According to prior art a radar cross section (RCS=radar cross section) of an object is referred to as the effective rescattering of the object, when it is iradiated from the outside. The radar cross section is in other words a measurement of how well the object is visible with a radar. To decrease the risk of an other radar to localize an object, for instance an antenna, it is required that its radar cross section is minimized.
Antennas are constructed to be used within particular specific frequency ranges, for instance about the 10 GHz frequency range. In this text, the working frequency of the antenna is defined as the frequency range for which the antenna is designed to be used.
In this text the threat frequency of the antenna is defined for which an incoming signal to the antenna constitutes a threat, when the incoming signal has a frequency which is in this frequency range. Below, the incoming signal is defined as the signal is within the threat frequency, because all other incoming signals to the antenna are not of interest, since they do not constitute a threat to the antenna.
Antennas according to prior art can comprise a normally flat disc, acting as ground plane and provided with antenna elements, which for instance can consist of through openings in the disc. Microwaves irradiate through the openings, when the openings are fed by a feeding network with microwave signals. The feeding network can consist of a wave guide. This is well known to the person skilled in the art.
Other types of elements can for instance be dipoles, wave guide openings, so-called horn and micro strip elements, so-called patches.
Other types of feeding networks can for instance comprise coaxial conductors, micro strip and strip line.
According to prior art most of the antennas of design reasons are provided with substantially uniform antenna element patterns, i. e., the antenna elements are arranged in a periodical pattern in the antenna, and the distances between the antenna elements are between one half and one whole wave length of the working frequency of the antenna.
When the openings in above disclosed disc are arranged in a periodical pattern in the disc, the micro wave signals, which normally are transmitted from the antenna elements, can cause strong constructive interference, i. e., co-operation of the microwave signals, in more directions than the intended head lobe direction. These strong interferences are termed grid lobes in the radiation diagram of the antenna.
At reception of signals in the antenna, the grid lobes correspondingly give a high sensitivity for signals incoming to the antenna from more directions than the direction of the head lobe.
Likewise, if the antenna from outside is irradiated with a signal having a frequency lying close to or above the working frequency of the antenna, grid lobes will arise also in particular directions in the radar radar cross section of the antenna, in which the beams reflected to the antenna in the signal co-operate with each other. These directions corresponds to a high value of reflection, i. e., the radar cross section (RCS) becomes large.
The arrangement of the grid lobes in space is determined by the distance between the antenna elements and which frequency the antenna works or is irradiated from outside. In the case where an antenna is employed for transmitting or receiving signals in a fixed direction, substantially perpendicular to the front area of the antenna, the minimum distance between the antenna elements can be slightly more than one wave length without grid lobes occurring in the radiation diagram of the antenna.
However, in the case an antenna is irradiated from outside of an to the antenna incoming signal, the distance between the antenna elements must be less than half a wave length of the incoming signal for grid lobes not occurring in the radar cross section of the antenna.
Depending on which threat frequency the antenna has, there will be different requirements on distances between the antenna elements (less than half a wave length of the threat frequency) for grid lobes not occurring in the radar cross section of the antenna, wherein it most often is the threat frequency of the antenna, which controls the packing density of the antenna elements in the antenna.
The existence of grid lobes in particular directions in the radar cross section of the antenna can easily be avoided totally if the antenna elements do not form a regular pattern in the antenna. In this case there will be no direction from the antenna elements within the radar cross section of the antenna in which the transmitted beams or the to the antenna reflected beams co-operate with each other, so a strong constructive interference occurs within the radar cross section of the antenna. However, because of design technical reasons, most of the antennas are provided with substantially regular antenna element patterns.
The packing density between the antenna elements of an antenna can according to prior art not be designed infinitely high of physical reasons. This is depending on that the working frequency of the antenna determines the dimension of the wave guides feeding the antenna elements. Thereby the wave guides cannot be made as small as possible, which delimits the distance between the antenna elements to a particular minimum distance. The result is that grid lobes occur in the radar cross section of the antenna if the antenna elements are arranged in a periodical pattern in the antenna, because the antenna elements cannot be packed infinitely dense.
A method according to prior art to decrease the distance between the antenna elements of an antenna is to use a material with a high dielectric constant, said material being arranged in the antenna elements and their feeding net work. The physical dimension of the antenna element and the feeding net work hereby becomes less and the packing density between the antenna elements becomes larger, whereby created grid lobes are moved up in the frequency band.
A drawback with the method described above is that the complexity is high in manufacturing of the antenna and the antenna is expensive to manufacture.
An other drawback is that the dielectric material normally implies increased losses.
In U.S. Pat. No. 5,461,392 an antenna system is disclosed comprising trays, in which a space between the trays is employed, in which space an attenuating material is arranged for reducing reflections of microwaves coming from the antenna system in the frequency range, where the attenuating material has an attenuating effect.
A disadvantage with this method is that grid lobes, as described above, occur in particular directions from the antenna system.
In U.S. Pat. No. 4,684,952 an antenna device is disclosed comprising a group of elements where feeding conductors to the elements are not connected to any transmitter or receiver, but the elements are only used for reflecting or absorbing signals. The elements can be connected to terminators, be short cut, or open conductors with phase shifters. In the latter cases, the signals the element receives can be retransmitted in desired direction. For instance, they can be retransmitted to the source of the incoming signal, in harmless direction, or be transmitted to a in front of the element group arranged feeder and thereby form a reflector antenna for transmission and reception of signals.